Acrylic Bearded Iris Painting

Bearded Iris- Acrylic Painting

I read on the internet that acrylic primer for walls can be used instead of gesso , which is the usual way of priming canvas for an oil painting. Since I happened to have a can of primer I thought I would use it on a few scrap pieces of mat board that I'm planning to use for painting some small acrylics.

And it worked pretty well, the acrylic paint was not absorbed into the mat but went on quite smoothly and allowed for blending. The result is this pair of purple bearded Irises -  painted in acrylic on 7 x 5 inch primed mat board.

Why use gesso when ordinary wall primer will do the job just as well without the higher price of gesso?